Joel George

Why PhD is not a super-sized Master's

Fri, 15 May 2026 00:00:00 +0530

PhD is commonly considered as the pinnacle of education — the highest degree and the final destination. Most people picture it as the grand finale — the end of all of that studying! But that is the wrong way to think about PhD for anyone considering this path. Think of a PhD less as graduation and more as admission — specifically, admission into the very beginning of an academic career. Think of it as the kindergarten of that world.

The traditional education system — with its syllabuses, timetables, and standardised exams — ends with your master’s degree. What comes next is a completely different game.

Making IC Engines Fast Enough for Quadrotor Control

Sat, 10 Jan 2026 00:00:00 +0530

Whether it’s military reconnaissance or wedding photography, using quadrotors (also known as multirotors) has become the standard. However, one of the primary limitations of today’s electric motor multirotors is their limited endurance, which is primarily due to battery capacity.

Using internal combustion (IC) engines would be much more efficient. While gasoline offers more than 25 times the energy density of lithium-polymer batteries, IC engines respond sluggishly compared to electric motors. Anyone who has driven a car with an IC engine knows the lag between pressing the accelerator and feeling the engine respond—a delay that would be catastrophic for a quadrotor trying to maintain stable flight. Quadrotors are inherently unstable flying machines. They stay airborne and level only because their electric motors can change speed in milliseconds, constantly adjusting thrust to maintain balance. Owing to delays in carburetors, fuel-air mixing, and combustion cycles, an IC engine seems fundamentally incompatible with the rapid response required to stabilize an IC engine-powered quadrotor. IC engine-based bi-rotor on a test stand

Equipping IC engines for quadrotor control is akin to transforming a marathon runner into a 100-meter sprint champion. That’s the engineering challenge Ajith, a PhD student whom I am co-guiding with Prof. Ramakrishna, tackled in his research, the results of which are published in an article titled “Throttle-controlled internal combustion engines as propulsion and control units for high endurance quadrotors: a feasibility study,” recently published in Aerospace Science and Technology.

Learning Aircraft Spin Dynamics from Noisy Flight Data

Fri, 12 Dec 2025 00:00:00 +0530

When an aircraft enters a spin—a motion wherein a stalled aircraft spirals downward—predicting its behavior becomes extraordinarily challenging. The aerodynamics are nonlinear and unsteady, and depend not only on what’s happening now but also on what happened moments before. Traditionally, researchers used extensive wind tunnel testing to build aerodynamic models for aircraft spin. These models are significantly more complex than the aerodynamic models required to predict flight during, for example, cruise or turn. Furthermore, this approach is time-consuming and expensive, often yielding models with limited fidelity.

Data-driven modeling offers a promising alternative, with techniques such as Dynamic Mode Decomposition (DMD) leading the way. However, these methods do not directly apply to real flight data, wherein the measurements of outputs as well as inputs (such as elevator deflection) are noisy. Also, the sensors used in aircraft have vastly different noise characteristics. Standard DMD methods fail to produce correct and reliable models in such cases. That’s exactly the problem my PhD student, Balakumaran, tackled in his research on robust aircraft spin modeling using enhanced Hankel Dynamic Mode Decomposition with error compensation—the results of which are published in the Aerospace journal.

Soft landings with hybrid rockets

Wed, 06 Nov 2024 00:00:00 +0530

In August 2023, Chandrayaan-3’s Vikram lander touched down softly on the lunar surface, demonstrating its precise landing technology. As we continue to celebrate this triumph, it’s exciting to consider how emerging technologies might shape the future of landings - both in space and on Earth.

Whether it’s a lunar lander gracefully descending on the Moon’s surface, a Mars explorer touching down on the Red Planet, or a cutting-edge vertical takeoff and landing (VTOL) aircraft on Earth, the ability to land softly and safely is a complex yet crucial challenge.

Historically, liquid rocket engines have been the go-to for achieving VTOL capabilities in planetary vehicles, largely due to their ability to provide controllable thrust. However, VTOL applications on Earth necessitate a safer option. Enter hybrid rockets, which could prove to be a superior alternative for VTOL operations within Earth’s atmosphere and in space, offering advantages that extend well beyond just safety.

In another post, I had described my PhD student Anandu Bhadran’s work on establishing the thrust controllability of hybrid rocket motors. Leveraging on that, we embarked on a journey to show that hybrid rocket motors can be used for soft landings. Beyond simulations, we wanted to demonstrate soft landing using hybrid rocket motors. However, we did not have the bandwidth, in terms of time and resources, to develop a complete platform for this.

Hence, Anandu, along with Prof. Ramakrishna and I, delved into an innovative approach to this problem – we demonstrated the practical feasibility of using hybrid rocket thrusters in landing platforms with a technique called hardware-in-the-loop simulation (HILS). We reported our studies in the International Journal of Aeronautical and Space Sciences.

This work was featured as a Linkedin post and also appeared in news.

Periodic orbits around asteroids

Fri, 30 Aug 2024 00:00:00 +0530

Asteroids, those ancient wanderers of our solar system, have long captivated astronomers and space scientists. Of course, the asteroids hold the key to unravelling the mysteries of our cosmic origins. But beyond that, these celestial bodies present unique opportunities for space exploration, and they do have some rare elements we would like to mine and take.

When it comes to exploring asteroids, finding the right orbits for spacecraft is crucial. Periodic orbits around these celestial bodies are potential trajectories for space probes, mining facilities, and even deep space stations. A periodic orbit is a closed, repeating trajectory in the asteroid’s rotating reference frame – like satellite trajectories around Earth. A periodic orbit family is a group of related periodic orbits with similar characteristics, such as shape, but may differ in size.

While various periodic orbit families and their bifurcations around asteroids have been extensively studied, a specific type of bifurcation, known as period-multiplying bifurcations, has received less attention. In his research published in Aerospace, Rishi—my PhD student—focused on computing these elusive period-multiplying bifurcations of periodic orbit families around asteroids.

The right multirotor for your microgravity experiment

Tue, 02 Apr 2024 00:00:00 +0530

Traditionally, microgravity research has been done in space stations or by dropping payloads from towers or balloons. These methods are great, but they can be expensive and have limited availability.

In a previous post, I talked about Siddhardha’s thesis that multirotors can be turned into microgravity platforms. In fact, he showed that every multirotor has the capability to be a microgravity platform. Multirotor microgravity platforms provide scientists with an affordable way to conduct experiments under near weightlessness conditions. But how do you choose the appropriate multirotor UAV for your microgravity experiment?

In a recent work published in Microgravity Science and Technology, Siddhardha and I laid down a framework for assessing the microgravity-producing capabilities of a multirotor UAV. Using our framework, you can estimate the g-time that a particular multirotor can provide while carrying the experimental setup of a specified weight.

Thrust control of hybrid rocket motors

Sun, 25 Jun 2023 00:00:00 +0530

Solid rocket motors and liquid rocket engines are the two primary forms of rocket propulsion. For example, GSLV has its first stage as solid motor, and all of its other propulsion systems, including the strap-ons are liquid engines.

A solid rocket motor consists of a casing with a solid propellent (fuel-oxidizer mixer) that burns to produce thrust. Liquid rocket engines store fuel and oxidizer as liquids which are mixed and burned in a combustion chamber and exhausted through a nozzle to produce thrust. Solid rocket motors are simple to manufacture and operate, but once ignited, cannot be controlled or stopped. Liquid rocket engines are complex, but the thrust produced can be controlled (including a complete shutdown and restart).

Although developed as early as solid and liquid rocket motors, a rocket propulsion technology that did not quite catch up with the other two is hybrid rocket motor which typically has a solid fuel and liquid/gaseous oxidizer.

Recently, hybrid rocket motors have gained increasing interest due to their unique characteristics that offer improved safety and reduced costs compared to traditional solid and liquid rocket motors. As a result, hybrid rocket motors are an area of active research and development, with the potential to revolutionize the field of rocket propulsion.

Modeling small UAV propellers

Sun, 23 Apr 2023 00:00:00 +0530

Propeller selection is a crucial stage in airplane design. A poor selection can result in an inefficient design.

Designers of small UAVs are often faced with a hurdle in the propeller selection stage in preliminary design due to the lack of simple yet accurate models to estimate small propellers’ performance (thrust coefficient, power coefficient, and efficiency at various combinations of forward speeds and propeller RPMs). It might even seem impossible to have accurate propeller performance models as the performance depends on the propeller geometry (airfoil characteristics, chord length, radius, and linear pitch). And small propellers have complex geometries, the details of which are proprietary and not publically available.

Nonetheless, we could still have accurate yet simple propeller models. That is what Siddhardha and I showed in our work published recently in Aerospace Systems.

Towards 'design and make in India' fixed-wing UAVs

Wed, 29 Mar 2023 00:00:00 +0530

Unmanned Aerial Vehicles (UAVs) have moved from the phase of ‘on paper’ applications to real-world applications.

In the early days of computers, people bought computers to use specific programs that came installed with the computer. Such software were called killer apps. Computers got sold for want of the killer apps.

UAVs are currently in that phase. UAV companies are selling their vehicles by advertising the specific application that their UAV is best at performing: for DJI, the killer app is drone photography; for Skydio, it is inspection, mapping, and survey; for Yamaha, it is precision agriculture; for many other companies, it is package delivery.

One of the recent applications of UAVs is in weather monitoring. In a previous post, I mentioned that one of the thrust areas of the Geophysical Flows Lab is using UAVs for field measurements. UAVs can acquire data with temporal and spatial resolutions that are missing in the data obtained using the current measurement systems.

Multirotors as microgravity platforms

Mon, 13 Mar 2023 00:00:00 +0530

If there is a fire breakout in the International Space Station (ISS), will the fire propagate as if on Earth?

There are these fantastic experiments done onboard ISS that reveal how physical phenomena behave differently under microgravity conditions. To study how physical and biological processes behave in microgravity conditions, we need to create microgravity. ISS naturally has microgravity, but access to ISS as an experimental platform is limited and expensive. A facility that allows us to simulate microgravity on Earth is a drop-tower — a tall tower from which the experimental set-up can be ‘dropped’ and the set-up experiences microgravity during the resultant free-fall. Building these tall drop-towers takes time and is costly.

Siddhardha, as part of his PhD thesis, proposed that multirotors can be turned into microgravity platforms. Thus, now we have a portable, cheap, and versatile microgravity platform.

Siddhardha’s work was featured as a news article in various newspapers.

Geophysical Flows Lab

Fri, 03 Mar 2023 00:00:00 +0530

Geophysical flows refer to the motion and behavior of fluids, such as air and water, in the Earth’s atmosphere and oceans. These flows are influenced by gravity, Earth’s rotation, temperature differences, and pressure gradients, and they exhibit a wide range of complex and nonlinear behavior. Geophysical flows can occur over a wide range of scales, from microscopic to planetary, and can have important implications for the Earth’s climate and weather patterns.

Examples of geophysical flows include ocean currents and atmospheric circulation. Understanding these flows is essential for predicting and mitigating natural hazards such as hurricanes, as well as for understanding the long-term dynamics of the Earth's climate and environment.

Petrola & Woods (2018)

The study of geophysical flows involves the application of (i) mathematical models, (ii) observational data, and (iii) laboratory experiments to understand better the underlying physical processes that govern the behavior of fluids in the Earth system. Geophysical Flows Lab, a Centre of Excellence at IIT Madras, is set up to make advances in the above three aspects with the vision of unlocking the mystery of Earth's cimalte mechanism.